fix(051/053): suite-scoped ${uri} (URL doubling) + unknown-id deletes are 404

Three suites (051_05, 053_05, 053_06) share the file-level variable ${uri} = /api/v1/context.jsonld and their setups Catenate the context server's host:port onto it, then Set GLOBAL Variable. In a full run the first suite publishes the absolute URL globally and the next suite's Catenate doubles it (http://0.0.0.0:8087http://0.0.0.0:8087/...) — every follow-up request 404s. 051_05's Catenate was even commented out, silently relying on whatever absolute URL another suite left behind. Each suite now catenates its own copy and uses Set Suite Variable.

051_04_01 (wrong id + reload=true) expected 400; clause 13.5.4 mandates ResourceNotFound for an identifier that 'does not correspond to any existing entry' — the reload bullets only describe behaviour for an existing @context. Corrected to 404.

The 051_05/053_05 LdContextNotAvailable 503→504 expectations are re-declared identically to their origin branch (unmerged MR) — git deduplicates on merge.

Analysis recorded as testsuite-doubts.md #97.